The present invention relates to a multi-beam feeding apparatus for a primary feed radiator of a reflector antenna to be mounted to a satellite.
Recently, as a demand for satellite communications and satellite broadcasting increases, a higher function for satellite antennas is required. For example, there are such important objects of future satellite antennas that a plurality of missions are shared with one antenna, a gain is increased in order to increase a capacity of communication, and an amount of communication in a plurality of beams is flexibly changed according to traffic conditions.
An example of such demands and a construction of the antenna thereto will be explained below.
FIG. 1 shows a model of a multi-beam satellite communication using four beams. In order to effectively use frequency resources, for example, the same frequency band is reused at intervals of two beams (in FIG. 1, beam #1 and beam #4 use the same the frequency band, respectively). In order to realize the frequency reuse as requirement, a technique that generates a low side lobe radiation pattern is important for Satellite antennas. For example, a radio wave radiated as a beam #1 is needed to have the low side lobe levels in its radiation pattern within this region in such a manner that the radio wave does not interfere with a coverage of a beam #4 using the same frequency band.
As an antenna apparatus for realizing such a low side lobe characteristic, the antenna comprising a reflector 11 and a primary radiator 12 is considered, as shown in FIG. 2.
As shown in FIG. 3A, the primary radiator 12 comprises sixteen element antennas T1 to T16, and each beam #1 to #4 is formed by a cluster comprising seven element antennas respectively, as shown in FIG. 3B. For example, the cluster forming the beam #1 comprises seven element antennas T1, T2, T5, T6, T7, T15 and T16 arranged around the element antenna T1.
Thus, one beam is formed by a plurality of element antennas and an appropriate excitation distribution is set relative to each element antenna, thereby it is possible to allow its synthesized pattern to have the low side lobe characteristic.
Furthermore, by the demand for enhancing a crossover level (the gain on a boundary between the beams), some of element antennas constituting the cluster are shared between the adjacent beams. For example, four element antennas T1, T2, T7 and T15 are shared between the cluster forming the beam #1 and the cluster forming the beam #2.
The above view is already used for many multi-beam antennas to be mounted to the satellite. A conventional system of the multi beam feeding apparatus thereof will be described below.
FIG. 4 is an example showing the construction of the conventional multi-beam feeding apparatus. Each element antenna T1 to T16 is connected to each beam forming network (BFN) 14-1 to 14-4 comprised for every beam #1 to #4 via filters 13-1 to 13-16 for removing an unnecessary radiation, respectively.
Each path of the element antennas T1 to T4, T7, T9, T10, T14 and T15 relating to a formation of a plurality of beams is connected to each diplexer 15-1 to 15-9. When a signal is transmitted, each diplexer 15-1 to 15-9 combines the signals from the beam forming network 14-1 to 14-4, respectively, and the combined signal is conducted to the corresponding element antennas T1 to T4, T7, T9, T10, T14 and T15, so that the signal is radiated to a space. Furthermore, when the signal is received, the received signals from the corresponding element antennas T1 to T4, T7, T9, T10, T14 and T15 are splitted for every frequency band, respectively, and the splitted signals are sent to the corresponding beam forming network 14-1 to 14-4.
The beam forming network 14-1 to 14-4 are so constructed that a divider (a combiner), a phase shifter and the like is used in a such a manner that a predetermined excitation distribution is provided to each element antenna constituting each cluster.
Each amplifier 16-1 to 16-4 for amplifying the signal of each beam #1 to #4 is connected to each beam forming network 14-1 to 14-4, respectively. As these amplifiers 16-1 to 16-4, when the signal is transmitted, a high power amplifier (concretely, a traveling-wave type amplifier tube, a solid-state power amplifier or the like) is used.
In this system, since the beam forming network 14-1 to 14-4 can flexibly set the excitation distribution of the element antennas T1 to T16, there is an advantage that an optimal low side lobe pattern for each beam can be realized for frequency reuse requirement.
However, there is a problem that this system becomes more complex according to the number of beams shared with the diplexers 15-1 to 15-9 connected to the element antennas T1, T2, T5, T6, T7, T15 and T16 shared with a plurality of beams #1 to #4.
Furthermore, in BFNs 14-1 to 14-4, it is necessary to set the excitation distribution considering the characteristics of the diplexers 15-1 to 15-9 connected to each antenna element T1 to T16. Accordingly, a design and the construction of BFNs 14-1 to 14-4 become more complex.
In addition, this system is constructed assuming that the frequency band of each beam #1 to #4 is fixed and is not changed. Therefore, the frequency band and the capacity of communication cannot be flexibly changed in response to the amount of calls of each beam #1 to #4. If the frequency band and the capacity of communication are attempted to be variable, instead of the diplexer, a combiner/divider is used. In this case, a flexibility relative to a variation of the capacity of communication between the beams can be obtained. However, on the other hand, a power loss occurs due to the combiner/divider, thereby there is another problem that a power efficiency is deteriorated and a heat is generated.